If a vehicular or other rechargeable battery is discharged beyond a certain state of depletion, it may be irreversibly overloaded and thus permanently disabled. In systems operating without charger, or even in charger-equipped vehicles where a faulty charger can cause great inconvenience, it is therefore desirable to monitor the state of charge of such a battery in order that the same may be recharged or, if need be, replaced in time.
To determine the state of charge of a battery in use, it is not sufficient to measure its terminal voltage since that voltage varies with the current drawn by the load. It is therefore necessary to derive a test parameter from simultaneous measurements of voltage and current with establishment of an alarm condition (e.g. the emission of a visual signal) by the measuring instrument upon ascertainment of a certain degree of depletion -- say, a discharge of 80% -- to warn the operator, usually with a certain delay in order to exclude transient conditions.
Instruments heretofore used for this purpose include rotary-coil galvanometers connected across a resistor lying in series with the load. In a moving vehicle, such galvanometers must be relatively insensitive to vibrations and therefore require a substantial input voltage for their operation so that the resistor must be of appreciable magnitude; such a series register, of course, dissipates considerable energy on being traversed by the load current. Thus, an accurate charge indication is difficult to obtain with conventional apparatus of this type.